As many as seven million Americans suffer from a form of hearing loss known as strial presbycusis, which is marked by a loss of hearing in all registers and, as the name indicates, is associated with the aging process. In a healthy ear there is a voltage difference across the basilar membrane, the organ that hosts the hair cells. This voltage difference, referred to as xe2x80x9cendocochlear potential,xe2x80x9d causes current to flow through the hair cells. Sound waves cause the hair cells to bend, thereby changing their electrical conductivity and the amount of current that flows through them. This process results in the electrical nerve impulses that are sent to the brain by the auditory nerve.
It appears that the most frequent immediate cause of strial presbycusis is the deterioration of the stria vascularis, a structure that extends along the basilar membrane and produces the ions that create the endocochlear potential. The loss of endocochlear potential appears to result in both an immediate decline in hearing acuity and a gradual deterioration of the structure of the scala media. One potential method of restoring the endocochlear potential is to inject additional charge by means of an electrode. This is difficult, however, because it requires the production of a DC current within the body. The body""s interstitial tissues tend to foul and eventually destroy any implanted electrode producing a DC current. Further, metal electrodes either dissolve or become plated with new material when they are driven with DC currents. Because of this, existing therapeutic devices which produce electrical currents within the body, including pacemakers and neural stimulation systems, are driven by charge balanced, biphasic electrical pulses.
In a first separate aspect, the present invention is a method to increase the endocochlear potential within the ear to restore normal hearing, comprising the implantation of an electrical device in the ear which maintains a voltage offset between the scala media and the surrounding tissue. The device is capable of injecting a unidirectional pulsatile current flow of at least an average of 5 xcexcA DC current into the scala media for at least thirty days.
In a second separate aspect, the present invention is a bodily implanted charge injection assembly for effecting a voltage difference between a specific body location and surrounding tissue. This assembly comprises a charge injection device that includes a first electrode assembly and a housing defining an interior space substantially enclosing the first electrode assembly and further defining a first opening placed near the specific location. The housing is filled with electrolytic solution. Also, a second electrode assembly is placed in contact with both the electrolytic fluid and the surrounding tissue. In addition, a physical gate assembly is adapted to selectively and controllably occlude the interior space so that either the first opening or the second electrode assembly may be occluded from the first electrode assembly. The physical gate assembly is controlled to drive current through the opening and alternately to refresh the first electrode assembly with current from the second electrode assembly.
In a third separate aspect the present invention is an electrolytic current injection device comprising an electrode and an electrolytic current port. In addition a control and rectification assembly is adapted to apply a biphasic pulse to said electrode, yet produce a pulsatile, unidirectional DC electrolytic current at said electrolytic current port.